This invention relates to a method of fabricating Group III nitride nanowires or nanocolumns that are vertically oriented with respect to a crystallographic template substrate, single-crystal, and free of threading dislocations.
The availability of substrate-bound high-density arrays of group III nitride columns or nanowires that are single crystal, free of threading dislocations, and vertically aligned with respect to the substrate would enable a wide range of applications in optoelectronics, electronics, field emitters, and sensors. Control of crystallographic orientation and growth direction are important properties for a method of fabricating suitable nanowires for many such applications.
Sapphire is the most common substrate for growing GaN heteroepitaxial films because it has the same crystal symmetry and is approximately lattice matched with GaN (lattice mismatch approximately 15% for c-plane sapphire and c-plane GaN). While this degree of lattice matching allows epitaxial growth, it also leads to a high density of threading dislocations in continuous GaN layers grown directly atop the sapphire substrate. The deleterious effects of threading dislocations on device performance in many applications have lead to the search for better alternative substrates. Alternative substrates that are sometimes used include substrates that are nearly lattice matched such as (111) γ-LiAlO2 (0.9% mismatch) and (111) MgO. Most conventionally grown GaN is c-plane GaN grown on (0001) sapphire (c-plane). The c-plane GaN is polar and columns of c-plane GaN ideally form hexagonal prisms or hexagon-derived shapes that exhibit angles characteristic the hexagonal structure. Other commercially available sapphire substrates include a-plane (11 20), m-plane (10 10), and r-plane (1 102) orientations. Nonpolar (11 20) a-plane GaN has been grown on (1 102) r-plane sapphire substrates, with [0001]GaN∥[ 1101]sapphire and [ 1100]GaN∥[11 20]sapphire (M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, “Nonpolar (11 20) a-plane Gallium nitride Thin films Grown on (1 102) r-plane Sapphire: Heteroepitaxy and Lateral Overgrowth,” Phys. Stat. Sol. (a), vol. 194 (2002). pp. 541-544). A selective epitaxy regrowth over a patterned SiO2 mask was used to produce low-threading dislocation material in the regrown regions over the SiO2 mask.
For applications where columnar GaN or GaN-based heterostructures including other nitrides such as AlN, AlGaN, AlGaInN, InGaN, and InN are required, direct growth of the columns on a suitable substrate without the use of a sacrificial template may be highly useful.
GaN nanowires have been reported using a technique where multi-layer Ni, Fe, or Au films (2-10 nm) are thermally evaporated onto silicon, c-plane sapphire, or a-plane sapphire and used as catalysts for metal-initiated metalorganic chemical vapor deposition. Metalorganic chemical vapor deposition (MOCVD) and substrate selection has been used to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires. Kuykendall and coworkers report gallium nitride nanowires synthesized via metal-initiated MOCVD. Wires were prepared on silicon, c-plane sapphire, and a-plane sapphire substrates. The wires were formed via the vapor-liquid-solid (VLS) mechanism with gold, iron, or nickel as growth initiators and were found to have widths of 15-200 nm. Transmission electron microscopy (TEM) showed that the wires were single-crystalline and were oriented predominantly along the [210] or [110] directions. Wires growing along the [210] orientation were found to have triangular cross-sections (T. Kuykendall, P. Pauzauskie, S. Lee, Y. Zhang, J. Goldberger, and P. Yang, “Metalorganic Chemical Vapor Deposition Route to GaN Nanowires with Triangular Cross Sections,” Nano Letters Vol. 3 (2003) pp. 1063-1066). The wires reported in this paper were not vertically oriented with respect to the substrate surface. Kuykendall and coworkers further report the epitaxial growth of wurtzite gallium nitride on (100) γ-LiAlO2 and (111) MgO single crystal substrates resulting in the selective growth of nanowires in the orthogonal [1 10] and [001] directions, exhibiting triangular and hexagonal cross-sections (T. Kuykendall, P. J. Pauzauskie, Y. Zhang, J. Goldberger, D. Sirbuly, J. Denlinger, and P. Yang, “Crystallographic alignment of high-density gallium nitride nanowire arrays,” Nature Materials Vol. 3 (2004) pp 524-528). The wires reported in this paper were vertically oriented with respect to the substrate surfaces, with triangular cross-section wires (15-40 nm wide and 1-5 micrometer long) grown on (100) γ-LiAlO2 and hexagonal cross-section wires grown on (111) MgO.
Lieber et al. U.S. Pat. No. 7,211,646 reports a method comprising growing a population of semiconductor nanowires catalytically from catalyst colloid particles. Laser vaporization of a composite target that is composed of a desired material (e.g. InP) and a catalytic material (e.g. Au) creates a hot, dense vapor which quickly condenses into liquid nanoclusters through collision with the buffer gas. Growth begins when the liquid nanoclusters become supersaturated with the desired phase and continues as long as the reactant is available. Growth terminates when the nanowires pass out of the hot reaction zone or when the temperature is turned down. Au is generally used as catalyst for growing a wide range of elongated nanoscale semiconductors. However, the catalyst is not limited to Au only. A wide rage of materials such as (Ag, Cu, Zn, Cd, Fe, Ni, Co . . . ) can be used as the catalyst. Generally, any metal that can form an alloy with the desired semiconductor material, but doesn't form more stable compound than the desired semiconductor with the elements of the desired semiconductor can be used as the catalyst. The key point of this process is that laser ablation generates liquid nanoclusters that subsequently define the size and direct the growth direction of the crystalline nanowires. The diameters of the resulting nanowires are determined by the size of the catalyst cluster, which in turn can be varied by controlling the growth conditions (e.g. background pressure, temperature, flow rate). For example, lower pressure generally produces nanowires with smaller diameters. Further diameter control can be done by using uniform diameter catalytic clusters. Laser ablation may be used as the way to generate the catalytic clusters and vapor phase reactant for growth of nanowires and other related elongated nanoscale structures, but fabrication is not limited to laser ablation. Many ways can be used to generate vapor phase and catalytic clusters for nanowire growth (e.g. thermal evaporation). Catalysts for LCG can be chosen in the absence of detailed phase diagrams by identifying metals in which the nanowire component elements are soluble in the liquid phase but that do not form solid compounds more stable than the desired nanowire phase; that is, the ideal metal catalyst should be physically active but chemically stable. From this perspective the noble metal Au should represent a good starting point for many materials. This laser-assisted catalytic growth (LGC) method of Lieber is readily extended to many different materials simply by producing solid targets of the material of interest and catalyst.
In the LGC method of Lieber, a pulsed laser is used to vaporize a solid target containing desired material and a catalyst, and the resulting liquid nanoclusters formed at elevated temperature direct the growth and define the diameter of crystalline nanowires through a vapor-liquid-solid growth mechanism. A key feature of this method is that the catalyst used to define 1D growth can be selected from phase diagram data and/or knowledge of chemical reactivity. A related approach termed solution-liquid-solid phase growth has been used by Buhro and coworkers to prepare nanowires of several III-V materials in solution, although not nitrides. LCG using a GaN/Fe target produces a high yield of nanometer diameter wire-like structures. A typical FE-SEM image of the product produced by LCG (FIG. 20A of Lieber et al., U.S. Pat. No. 7,211,464) shows that the product consists primarily of 1D structures with diameters on the orders of 10 nm and lengths greatly exceeding 1 micrometer. (The wires in FIG. 20A are not vertically oriented with respect to a substrate, but appear randomly distributed thereon and recumbent.) However, Au exhibits poor solubility of N and thus may not transport N efficiently to the liquid/solid growth interface. Consistent with this analysis, Lieber et al. reported being unable to obtain GaN nanowire using the Au catalyst.
The patent of Cuomo and coworkers (J. J. Cuomo, N. M Williams, A. D. Hanser, E. P. Carlson, and D. T. Thomas, U.S. Pat. No. 6,692,568) reports a method utilizing sputter transport techniques to produce arrays or layers of self-forming, self-oriented columnar structures characterized as discrete, single-crystal Group III nitride posts or columns on various substrates. The columnar structure is formed in a single growth step, and therefore does not require processing steps for depositing, patterning, and etching growth masks. A Group III metal source vapor is produced by sputtering a target for combination with nitrogen supplied from a nitrogen-containing source gas. The III/V ratio is adjusted or controlled to create a Group III metal-rich environment within the reaction chamber conducive to preferential column growth.
Wang and coworkers have reported the growth of well aligned, vertically oriented GaN nanowires on (1 102) r-plane sapphire wafers via metal-organic chemical vapor deposition using nickel nitrate catalyst. This paper reports that the degree of alignment and size uniformity of the nanowires is highly dependent on the nickel nitrate catalyst concentration used. This paper (G. T. Wang, A. A. Talin, D. J. Werder, J. R. Creighton, E. Lai, R. J. Anderson, and I. Arslan, “Highly aligned, template-free growth and characterization of vertical GaN nanowires on sapphire by metal-organic chemical vapour deposition,” Nanotechnolotgy, vol. 17 (2006) pp 5773-5780) is incorporated herein by reference.